It is known, in particular for Mach-Zehnder electro-optic modulators, for it to be necessary in certain applications to make periodic adjustments to biasing of the modulator to ensure that it continues to operate at or near a quadrature point. There are various reasons why during operation of a Mach-Zehnder electro-optic modulator a given modulating voltage level will result in a slightly inconsistent optical output level from the modulator. That is, the bias point—the voltage that needs to be applied to a bias electrode for example in order to return the modulator to or near a quadrature point—is prone to drift over time. This may be due to trapped charges in the modulator waveguide, temperature variations or other effects.
Temperature variations in particular are likely to result in physical changes to the length of the interferometer in a Mach-Zehnder modulator for example, with consequent variations in the bias point. There are certain types of Mach-Zehnder Interferometer (MZI) optical modulator which have a bias point that remains electrically stable over time (i.e. no trapped charge or similar drift effects). Such a modulator is still, however, temperature-sensitive: any variation in temperature changes the length of the interferometer. As the interferometer in a Mach-Zehnder modulator is asymmetric, the length of the asymmetry is also altered with varying temperature and therefore the bias point shifts, requiring an adjustment to an applied dc bias voltage to correct it.